Abstract
We evaluate the frictional strength of seismogenic faults in the Main Ethiopian Rift (MER) by inverting the available, well-constrained earthquake focal mechanisms. The regional stress field is given by − 119.6°/77.2°, 6.2°/7.6°, and 97.5°/10.2° for trend/plunge of σ1, σ2 and σ3, respectively agrees well with previous fault kinematic and focal mechanism inversions. We determine the coefficient of friction, μ, for 44 seismogenic faults by assuming the pore pressure to be at hydrostatic conditions. Slip on 36 seismogenic faults occurs with μ ≥ 0.4. Slip on the remaining eight faults is possible with low μ. In general, the coefficient of friction in the MER is compatible with a value of μ of 0.59 ± 0.16 (2σ standard deviation). The shear stresses range from 16 to 129 MPa, is similar to crustal shear stress observed in extensional tectonic regimes and global compilations of shear stresses from major fault zones. The maximum shear stress is observed in the ductile crust, below the seismologically determined brittle-ductile transition (BDT) zone. Below the BDT, the crust is assumed to be weak due to thermal modification and/or high pore fluid pressure. Our results indicate linearly increasing μ and shear stress with depth. We argue that in the MER upper crust is strong and deforms according to Coulomb frictional-failure criterion.
Highlights
Thorough understanding of the regional stress field is of paramount importance in constraining the strength of faults and the crust along actively deforming plate boundaries
We showed that the Main Ethiopian Rift (MER) faults are favorably oriented and facilitate the easy passage of fluids and supports the notion that the pore pressure in the region is near hydrostatic state
Since most of focal mechanisms occur at depths above 16 km, the assumption of hydrostatic pore pressure can be considered as adequately representative of the state of pore fluid pressure in the upper crust and well depicts the strength of the crust and faults in the MER
Summary
Thorough understanding of the regional stress field is of paramount importance in constraining the strength of faults and the crust along actively deforming plate boundaries. Numerical modeling studies in active tectonic areas, e.g. in East African Rift (EAR) (Bird et al, 2006), argue that the friction coefficient is much lower than the laboratory estimates. This agrees with recent laboratory experiment that found μ to be below 0.4 under pressure condition equivalent to a depth of ∼15 km (Di Toro et al, 2011). The results presented will contribute to the understanding of the stress magnitude at the earthquake focal depths where in situ measurements are totally absent and the frictional strength of MER faults and crust
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